Tag Archive | "precision"

Project Review – Silicon Chip Capacitance Substitution Box

Introduction

Every month Australian electronics magazine Silicon Chip publishes a variety of projects, and in some cases various (well … one of two) electronics retailers will pick up the project and offer it as a kit. However for an increasing number of new projects they don’t, which leaves the interested reader with one option – build the entire project from scratch.

But thankfully this is no longer the case – as the team from Silicon Chip now offer a range of project PCBs and matching front panels for sale directly from their website. Although buying these parts is not the cheapest option, it gives the busy person who likes making things a quick start – or the inexperienced more opportunities to complete a successful project.

So as a test of this new service, I bought the PCB and front panel for the Capacitance Substitution Box project described by Nicholas Vinen in the Juily 2012 issue of SC:

capacitance_article

This is something I’ve meant to make for a while – but didn’t really have the inclination to make one from scratch, so it was neat to see a version published in the magazine. I believe the subjects in the magazine article are oftern prototypes, which explains the difference in colour for the front panel.

The parts arrived in a week after placing the order, and are of a high quality:

capacitance box panel

capacitance pcb front

capacitance pcb rear

When complete, the capacitance substitution box PCB and panel will fit nicely into an Altronics H0151 enclosure, so you don’t need to do any drilling or filing. The next task was to organise the required parts. The rotary switches, terminal posts and the usual odds and ends can be found at Altronics, Jaycar or other suppliers. However the main components – the capacitors – offered two options.

The first option is to simply use capacitors from personal stock or the stores. However the tolerance of these parts can vary wildly, with up to twenty percent either way. This is ok for simple uses, however when values are combined – the tolerance of larger values can negate the lower values completely. So instead I’ve chosen the second option – which involves using brand-name low-tolerance capacitors.

Thus I turned to element14 who stock not only a huge range of not only regular but also the low-tolerance capacitors, and can also have them on my desk usually by the next working day. Finally, it’s nice to have all the parts arrive in little bags… neatly organised ready to go:

capacitors

It’s easy to search for low-tolerance parts with element14, as the automatic filtering has tolerance as a parameter:

element14 capacitors

Furthermore you can also ensure you have the voltage rating of at least 50V DC as well. So after half an hour the capacitor order was completed and arrived when expected – using parts from Panasonic, Vishay, and Wima. The tolerances of our capacitors used varied between one and ten percent, which will help improve the accuracy of the substitution box.

Assembly

The PCB has the capacitor values labelled neatly on the silk-screen, so soldering in all the capacitors was a relatively simple but long operation. Having them arrive in separate packets made life a lot easier. During the soldering process it’s a good idea to have a  break or two, which helps you avoid fatigue and making any mistakes.

capacitance substitution box half finished

There may be a few capacitors that are a little too wide to fit with the others, so they can be mounted on the other side of the PCB:

capacitance substitution box wide capacitor

However they all end up fitting well:

capacitance substitution box half finished

The next step was to configure the first rotary switch for six position use, then cut the plastic stopped from the side of each rotary switch. In the following image you have a before and after example:

capacitance substitution box rotary switches

Now the rotary switches can have their shafts trimmed and then be soldered onto the PCB:

capacitance substitution box switches trimmed

However ensure you have the first rotary switch in the right way – that is the selections are selected across the top half, not the bottom. Remove the nuts from the rotary switches, and double-check all the capacitors are fitted, as once the next step is completed … going back will be difficult to say the least.

At this point the banana sockets can be fitted to the panel, and then soldered into place, and then you’re finished. Just place the panel/PCB combination inside the box and screw it down:

capacitance substitution box complete

Using the Capacitance Substitution Box

Does it work? Yes – however you don’t get exact values, there will always be a tolerance due to the original tolerance of the capacitors used and the stray capacitance of the wires between the box and the circuit (or capacitance meter). Nevertheless our example was quite successful. You can see the box in action with our Altronics LC meter kit in this video.

Again, using the best tolerance capacitors you can afford will increase the accuracy of this project.

Conclusion

Over time this would be a useful piece of equipment to have – so if your experiments or projects require varying capacitor value, this project will serve the purpose nicely. Plus it helps with mental arithmetic and measures of capacitance! Please do not ask me for copies of the entire Silicon Chip article, refusal may offend. Instead – visit their website for a reprint or digital access.

And if you enjoyed this article, or want to introduce someone else to the interesting world of Arduino – check out my book (now in a third printing!) “Arduino Workshop”.

visit tronixlabs.com

Have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our forum – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Posted in capacitor, kit review, projects, silicon chip, substitution box

Kit review – High Accuracy LC Meter

Hello readers

Time for another kit review. Lately one of my goals has been to make life easier and in doing so having some decent test equipment. One challenge of meeting that goal is (naturally) keeping the cost of things down to a reasonable level. Unfortunately my eyesight is not the best so I cannot read small capacitor markings – which makes a capacitance meter necessary. Although I have that function within my multimeter, it is often required to read resistors in the same work session.

Thus the reason for this kit review – the High Precision LC Meter kit. The details were originally published in the May 2008 issue of Australia’s Silicon Chip magazine. The meter specifications are:

  • Capacitance – 0.1pF to over 800 nF with four-digit resolution;
  • Inductance – 10 nH to over 70 mH with four-digit resolution;
  • Accuracy of better than +/- 1% of the reading;
  • Automatic range selection, however only non-polarised capacitors can be measured.

The power drain is quite low,  between 8 (measurement) and 17 milliamps (calibration). Using a fresh 9V alkaline battery you should realise around fifty to sixty hours of continuous use. At this point some of you may be wondering if it is cheaper to purchase an LC meter or make your own. A quick search found the BK Precision 875B LCR meter with the same C range and a worse L range for over twice the price of the kit. Although we don’t have resistance measurement in our kit, if you are building this you already have a multimeter. So not bad value at all. And you can say you built it 🙂

Speaking of building, assembly time was just under two hours, and the kit itself is very well produced. The packaging was the typical retail bag:

retailkitss

The first thing that grabs your attention is the housing. It is a genuine, made in the US Hammond enclosure – and has all the required holes and LCD area punched out, so you don’t need to do any drilling at all:

hammondcasess

The enclosure has nice non-slip rubberised edging (the grey area) and also allows for a 9V battery to be housed securely. The team at Altronics have done a great job in redesigning the kit for this enclosure, much more attractive than the magazine version. The PCB is solder-masked and silk-screened to fine standard:

pcbss2

There are two small boards to cut and file off from the main PCB. We will examine them later in the article. All required parts for completion were included, and it is good to see 1% resistors and an IC socket for the microcontroller:

partsss1

At first I was a little disappointed to not have a backlit LCD module, however considering the meter is to be battery operated (however there is a DC socket for a plugpack) and you wouldn’t really be using this in the dark, a backlight wouldn’t be necessary. Construction was easy enough, the layout on the PCB is well labelled, and plenty of space between pins. Lately I have started using a lead-former, and can highly recommend the use of one:

leadformerss

Assembly was quite simple, just start with the lower profile components:

assemble1ss

 

… then mount the LCD and the larger components:

assemble2ss

… the switches and others – and we’re done:

finishedsolderingss

The only problem at this point was the PCB holes for the selector switch, one hole was around 1mm from where it needed to be. Instead of drilling out the hole, it was easier to just bend up the legs of the switch and keep going:

switchlegsss

At this stage one has to cut out two supports from the enclosure, which can be done easily. Then insert the PCB and solder to the sockets and power (9V battery snap). Initial testing was successful (after adjusting the LCD contrast…

inittestss

If you look at the area of PCB between the battery and the left-hand screw there are eight pins – these are four pairs of inputs used to help calibrate and check operation of the meter. For example, by placing a jumper over a pair you can display the oscillator frequency at various stages:

calibrationss

Furthermore, those links can also be used to fine-tune the meter. For example one can increase or decrease the scaling factor and the settings are then stored in the EEPROM within the microcontroller. However my example seemed ok from the start, so it was time to seal up the enclosure and get testing. Starting with a ceramic capacitor, the lowest value in stock:

3p9pfss

Spot-on. That was a good start, however trying to bend the leads to match the binding posts was somewhat inconvenient, so I cut up some leads and fitted crocodile clips on the end. The meter’s zero button allows you to reset the measurement back to zero after attaching the leads, so stray capacitance can be taken into account.

Next, time to check the measurement with something more accurate, a 1% tolerance silvered-mica 100 picofarad capacitor:

99pfss

Again, the meter came through right on specification. My apologies to those looking for inductor tests – I don’t have any in stock to try out. If you are really curious I could be persuaded to order some in, however as the capacitance measurement has been successful I am confident the inductance measurement would also fall within the meter’s specifications.

As shown earlier, there were two smaller PCBs included:

pcbadaptorsss

The top PCB is a shorting bar used to help zero the inductance reading, and the lower PCB is used to help measure smaller capacitors and also SMD units. A nice finishing touch that adds value to the meter. The only optional extra to consider would be a set of short leads with clips or probes to make measurement physically easier.

When reading this kit review it may appear to be somewhat positive and not critical at all. However it really is a  good instrument, considering the accuracy, price, and enjoyment from doing it yourself. It was interesting, easy to build, and will be very useful now and in the future. So if you are in the market for an LC meter, and don’t mind some work – you should add this kit to your checklist for consideration. It is available from our store – Tronixlabs.com

 

visit tronixlabs.com

… which along with being Australia’s #1 Adafruit distributor, also offers a growing range and Australia’s best value for supported hobbyist electronics from DFRobot, Freetronics, Seeedstudio and much much more.

As always, have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our forum – dedicated to the projects and related items on this website.

Posted in K2533, kit review, LC meter, test equipment, tronixlabsComments (18)

The 555 Precision Timer IC

Learn about the useful and inexpensive 555 timer IC in this detailed tutorial!

Hello readers

Today we revisit one of the most popular integrated circuits ever conceived – the 555 timer IC. “Triple-five”, “five-five-five”, “triple-nickel” … call it what you will, it has been around for thirty-eight years. Considering the pace of change in the electronics industry, the 555 could be the constant in an ever-changing universe. But what is the 555? How does it work? How can we use it? And … why do we still use it? In this introductory article we will try to answer these questions. If you would like to see some examples, visit here.

What is the 555?

The 555 timer is the solution to a problem found by the inventor – Hans Camenzind.  He saw the need through his radio work for a part that could act as an oscillator or a timer [1]; and working as a contractor for Signetics developed the 555. (Signetics was purchased by Philips in 1975, and their semiconductor division was spun off as NXP in 2006). The 555 has to be one of the most used ICs ever invented. It is used for timing, from microseconds to hours; and creating oscillations (which is another form of timing for the pedants out there). It is very flexible with operation voltage, you can throw from 4.5 to 18V at it; you can sink or source 200mA of current through the output; and it is very cheap – down to around nine cents if you order several thousand units. Finally, the 555 can achieve all of this with a minimum of basic components – some resistors and capacitors.

Here are some examples in the common DIP casing:

555sss

Furthermore a quick scan of suppliers’ websites show that the 555 is also available in surface-mount packages such as SOIC, MSOP and TSSOP. You can also source a 556 timer IC, which contains two 555 ICs. (What’s 555 + 555? 556…) Furthermore, a 558 was available in the past, but seems rather tricky to source these days.

556sss

How does the 555 work?

The 555 contains two major items:

  • A comparator – a device which compares two voltages, and switches its output to indicate which is larger, and
  • A flip-flop – a circuit that has two stable states, and those states can be changed by applying a voltage to one of the flip-flop’s inputs.

Here is the 555 functional diagram from the TI 555 data sheet.pdf:

functiondiagram

… and the matching pin-out diagram:

Don’t let the diagrams above put you off. It is easier to explain how the 555 operates within the context of some applications, so we will now explore the three major uses of the 555 timer IC in detail – these being astable,  monostable, and bistable operations, in theory and in practice.

Astable operation

Astable is an on-off-on… type of oscillation – and generates what is known as a square wave, for example:

sqwaveastable

There are three values to take note of:

  • time (s) – the time for a complete cycle. The number of cycles per second is known as the frequency, which is the reciprocal of time (s);
  • tm (s) – the duration of time for which the voltage (or logic state) is high;
  • ts (s) – the duration of time for which the voltage (or logic state) is low.

With the use of two resistors and one capacitor, you can determine the period durations. Consider the following schematic:

555astableschematic

Calculating values for R1, R2 and C1 was quite simple. You can either determine the length of time you need (t) in seconds, or the frequency (Hz) – the number of pulses per second.

t (time) = 0.7 x (R1 + [2 x R2]) x C1

f (frequency) = 1.4 / {(R1 + [2 x R2]) x C1}

Where R1 and R2 are measured in ohms, and C1 is measured in farads. Remember that 1 microfarad = 1.0 × 10-6 farads, so be careful to convert your capacitor values to farads carefully. It is preferable to keep the value of C1 as low as possible for two reasons – one, as capacitor tolerances can be quite large, the larger the capacitor, the greater your margin of error; and two, capacitor values can be affected by temperature.

How the circuit works is relatively simple. At the time power is applied, the voltage at pin 2 (trigger) is less than 1/3Vcc. So the flip-flop is switched to set the 555 output to high. C1 will charge via R1 and R2. After a period of time (Tm from the diagram above) the voltage at pin 6 (threshold) goes above 2/3Vcc. At this point, the flip-flop is switched to set the 555 output to low. Furthermore, this enables the discharge function – so C1 will discharge via R2. After a period of time (Ts from the diagram above) the voltage at pin 2 (trigger) is less than 1/3Vcc. So the flip-flop is switched to set the 555 output to high… and the cycle repeats.

Now, for an example, I want to create a pulse of 1Hz (that is, one cycle per second). It would be good to use a small value capacitor, a 0.1uF. In farads this is 0.0000001 farads. Phew. So our equation is 1=1.4/{(R1 + [2 x R2]) x C1}. Which twists out leaving us R1=8.2Mohm, R2=2.9MOhm and C1 is 0.1uF. I don’t have a 2.9MOhm resistor, so will try a 2.7MOhm value, which will give a time value of around 0.9s. C2 in astable mode is optional, and used if there is a lot of electrical noise in the circuit. Personally, I use one every time, a 0.01uF ceramic capacitor does nicely. Here is our example in operation:

Notice how the LED is on for longer than it is off, that is due to the ‘on’ time being determined by R1+R2, however the ‘off’ time is determined by R2 only. The ‘on’ time can be expressed as a percentage of the total pulse time, and this is called the duty cycle. If you have a 50% duty cycle, the LED would be on and off for equal periods of time. To alter the duty cycle, place a small diode (e.g. a 1N4148) over pins 7 (anode) and 2 (cathode). Then you can calculate the duty cycle as:

Tm = 0.7 x R1 x C1 (the ‘on’ time)

Ts = 0.7 x R2 x C1 (the ‘off’ time)

Furthermore, the 555 can only control around 200mA of current from the output to earth, so if you need to oscillate something with more current, use a switching transistor or a relay between the output on pin 3 and earth. If you are to use a relay, put a 1N4001 diode between pin 3 (anode) and the relay coil (cathode); and a 1N418 in parallel with the relay coil, but with the anode on the earth side. This stops any reverse current from the relay coil when it switches contacts.

Monostable operation

Mono for one – one pulse that is. Monostable use is also known as a “one-shot” timer.  So the output pin (3) stays low until the 555 receives a trigger pulse (drop to low) on pin 2. The length of the resulting pulse is easy to calculate:

T = 1.1 x R1 x C1;

where T is time in seconds, R1 is resistance in ohms, and C1 is capacitance in farads. Once again, due to the tolerances of capacitors, the longest time you should aim for is around ten minutes. Even though your theoretical result for T might be 9 minutes, you could end up with 8 minutes 11 seconds. You might really need those extra 49 seconds to run away…  Though you could always have one 555 trigger another 555… but if you were to do that, you might as well use a circuit built around an ATmega328 with Arduino bootloader.

Now time for an example. Let’s have a pulse output length of (as close as possible to) five seconds. So, using the equation, 5 = 1.1 x R1 x C1… I have a 10 uF capacitor, so C1 will be 0.00001 farads. Therefore R1 will be 454,545 ohms (in theory)… the closest I have is a 470k, so will try that and see what happens. Note that it you don’t want a reset button (to cancel your pulse mid-way), just connect pin 4 to Vs. Here is the schematic for our example:

555monostable

How the monostable works is quite simple. Nothing happens when power is applied, as R2 is holding the trigger voltage above 1/3Vcc. When button S1 is pushed, the trigger voltage falls below 1/3Vcc, which causes the flip-flop to set the 555’s output to high. Then C1 is charged via R1 until the threshold voltage 2/3Vcc is reached, at which point the flip-flip sets the output low and C1 discharges. Nothing further happens until S1 is pressed again. The presence of the second button S2 is to function as a reset switch. That is, while the output is high the reset button, if pressed, will set the output low and set C1 to discharge.

Below is a video of my example at work. First I let it run the whole way through, then the second and subsequent times I reset it shortly after the trigger. No audio in clip:

Once again, we now have a useful form of a one-shot timer with our 555.

Bistable operation

Bistable operation is where the 555′s output is either high, or low – but not oscillating. If you pulse the trigger, the output becomes and stays high, until you pulse reset. With a bistable 555 you can make a nice soft-touch electronic switch for a project… let’s do that now, it is so simple you don’t need one of my quality schematics. But here you are anyway:

555bistablesch

In this example. pressing S1 sets the voltage at pin 2 (trigger) to below 1/3Vcc, thereby setting the output to high – therefore we call S1 our ‘on’ switch. As pin 6 (threshold) is permanently connected to GND, it cannot be used to set the output to low. The only way to set the output back to low is by pressing S2 – the reset button, which we can call the ‘off’ switch. Couldn’t be easier, could it? And that output pin could switch a transistor or a relay on or off, who knows? Your only limit is your imagination. And here’s one more video clip:

And there you have it – three ways in which we can use our 555 timer ICs. But in the year 2011, why do we still use a 555? Price, simplicity, an old habit, or the fact that there are so many existing designs out there ready to use. There will be many arguments for and against continued use of the 555 – but as long as people keep learning about electronics, the 555 may still have a long and varied future ahead of it.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

References

[1] “The 555 Timer IC – An interview with Hans Camenzind” (Jack Ward – semiconductormuseum.com)

Various diagrams and images from the Texas Instruments NE555 data sheet.

Posted in 555, clocks, COM-09273, electronics, LCD, lesson, tronixstuff, tutorial, xbeeComments (16)


Subscribe via email

Receive notifications of new posts by email.

The Arduino Book

Arduino Workshop

Für unsere deutschen Freunde

Dla naszych polskich przyjaciół ...

Australian Electronics!

Buy and support Silicon Chip - Australia's only Electronics Magazine.

Use of our content…

%d bloggers like this: